# UNDF: UNDF-2026-000000728
--- a/nova/scheduler/host_manager.py
+++ b/nova/scheduler/host_manager.py
@@ -510,7 +510,7 @@ class HostManager(object):
         def _match_forced_hosts(host_map, hosts_to_force):
             forced_hosts = []
-            lowered_hosts_to_force = [host.lower() for host in hosts_to_force]
+            lowered_hosts_to_force = {host.lower() for host in hosts_to_force}
             for (hostname, nodename) in list(host_map.keys()):
                 if hostname.lower() not in lowered_hosts_to_force:
                     del host_map[(hostname, nodename)]